Filters and the like that use a filter media bed to remove solids from liquids are well known. Such systems typically include a liquid distribution system that collects liquid after it is filtered in one flow direction and distributes clean liquid through the filter media in a reverse flow direction to effect a cleaning process known as backwashing.
Water filtration systems of the gravity type are commonly employed for filtering high volumetric flow rates of liquids, e.g., in municipal and industrial water treatment and waste water treatment plants. Filtration systems generally comprise one or more filters, each employing a bed of granular filter media for filtering a liquid as it seeps downward through the filter bed.
Each filtration system generally comprises an open filter basin having a floor and vertical walls surrounding the floor and an underdrain positioned over the floor. The underdrain defines a perforated false bottom in the basin for supporting the filter media bed and to provide a system of fluid passageways for both removing the filtered water from the bottom of the filter basin and directing water and/or air into the filter bed during backwashing.
The filter media bed is generally several feet deep and is typically comprised of successive layers of gravel, sand, anthracite, or other granular filter media. Traditional filter designs use support media such as multiple gravel layers, beginning with relatively coarse sizes next to the underdrain's top surface and gradating up to relatively fine sizes, are placed on the top surface of the underdrain to prevent the finer filter media from entering the underdrain and contaminating the filtered water.
Other filter designs are considered "gravel-less" and use various types and configurations of porous media to prevent the granular filter media from entering the underdrain. The filter media for both traditional and gravel-less designs consist of one or more layers of sand, anthracite, or other filter media, gradated from coarse on top to fine on bottom, and placed upon the support gravel or porous media.
During operation of the filtration system, the influent, i.e., unfiltered water, is directed into the filter basin to a depth of several feet above the upper layer of filter media. The influent is allowed to flow downward though the filter media bed. During this process, the suspended materials in the unfiltered water become trapped in the filter media. The water ultimately reaches the bottom of the filter bed and passes through the perforations in the underdrain system. The water is then collected in a system of fluid passageways within the underdrain system and is carried out of the filter basin through a suitable conduit or flume.
After the filtration system is operational for an extent of time, the efficiency of the system decreases and it becomes necessary to wash the filter media bed to remove material trapped therein. Washing of the filter media is accomplished by utilizing a backwashing process. The backwashing process involves pumping pressurized water and/or air in a reverse direction into the system of fluid passageways in the underdrain system upward through the perforations in the underdrain, and into the overlying filter media bed. The wash water flowing upwardly through the filter media bed carries the trapped materials upward from the filter bed. The wash water and the materials entrained or suspended therein are then collected at the top of the filter basin and carried away.
During the backwashing operation it is desirable to obtain a uniform distribution of wash water throughout the filter media bed to effect complete washing of the entire filter bed. If the wash water distribution is uneven so that dead spots occur at certain locations within the filter bed, then those portions of the filter bed will be improperly cleansed, thereby reducing the efficiency of the filter.
The backwashing process must also be performed under carefully controlled conditions so as to avoid unduly disturbing or damaging the filter media bed. For example, the velocity of the wash water must be controlled at a level below that which would cause the filter media to become entrained in the wash water along with the removed materials and carried away as waste. "Blow holes," in which explosive bursts of wash water open channels in the filter media at the initiation of the backwashing cycle, must also be avoided. These blow holes allow influent to pass through the filter media without being filtered and allow finely-sized filter media to be carried away with the effluent, i.e., the filtered water.
Several underdrain designs have been developed over the years to address some of these potential problems. A common type of underdrain utilizes the multi-block, modular design in which approximately two- to four-foot long blocks, typically made of either ceramic, cement or plastic, are laid end to end and disposed next to each other in parallel rows, and then cemented or grouted in place to form the underdrain.
The interior of a typical block is divided into upper and lower chambers, i.e., horizontal passageways, separated by a horizontal partition or lateral, but interconnected by a plurality of orifices formed through the lateral. The multi-block, modular design is commonly employed in a filter having a central flume extending below the filter bottom, formed through the concrete supporting structure. The blocks that are vertically aligned with the flume have at least a portion of their bottom walls removed. Thus, the lower chambers are in fluid communication with the flume through orifices or cut-outs.
Modular underdrain designs inherently have joints to connect the blocks end-to-end. These joints may be prone to leakage, both external to the block, or internally, between the various chambers, thus inconsistently varying the hydraulic characteristics of the underdrain.
Most prior art joints designs also induce head loss across the joints, which further adversely affects the hydraulic characteristics of the underdrain. This head loss increases the pressure drop down the length of the underdrain and therefore requires a relatively larger chamber cross-sectional area to maintain favorable distribution characteristics. The larger chamber required for this type of underdrain results in a larger overall height and the height of the underdrain directly affects the required depth of the filter and associated costs. Thus, it is desirable to provide an underdrain system which does not have or need joints.
Prior art underdrains typically utilize porous filter media or caps which are situated on top of the underdrain system to serve as an additional filtering mechanism before the influent enters the underdrain. These caps, preferably porous plates, are usually screwed into place on the top of the underdrain or held in place with the use of gaskets. Either way, installation of the porous plates is rather difficult and inefficient, making it a time-consuming and costly procedure. Maintenance is thus overly burdensome as well, as over periods of use, the porous plates need to be removed and replaced. Prior art techniques also often result in imperfect seals between the underdrain and the porous plates. Thus, it is desirable to provide an underdrain system which provides for improved sealing, a more efficient manner of installation and reduced maintenance of porous filter media.
In addition, many prior art designs are very heavy structures, which makes shipment and installation difficult and costly. Thus, it is desirable to provide an underdrain system that comprises relatively light components, making it easier to transport, install, assemble, and maintain the system.
In general, prior art modular underdrains are very complex, typically consisting of many parts. This complexity, from which many drawbacks result, creates significantly high production costs as well as high costs of labor in installing and maintaining the many blocks of modular underdrain systems. As a primary object of the present, it is thus desirable to provide an underdrain that comprises less components and is relatively simple in its design, making it relatively inexpensive to manufacture, install and maintain.